As is conventionally known, an exhaust gas purification device is provided in an exhaust flow passage of an internal combustion engine in order to collect particulate (particulate substance) in exhaust gas discharged from an internal combustion engine and to reduce the amount of NOx therein.
As an exhaust gas purification device for collecting particulate, there has been developed one equipped with a diesel particulate filter (hereinafter referred to as DPF), and, as an exhaust gas purification device for achieving a reduction in NOx amount, there has been developed one equipped with a DeNOx catalyst such as a NOx reduction catalyst or a NOx occlusion reduction catalyst.
In recent years, the exhaust gas regulations are becoming more and more strict. To cope with this, there has been proposed an exhaust gas purification device in which a DPF and a DeNOx catalyst are combined in series. In such an exhaust gas purification device, particulate is collected by the DPF on the input side, which is, for example, on the upstream side, and it is possible to achieve a reduction in NOx by the DeNOx catalyst on the output side, which is on the downstream side thereof, thus making it possible to achieve a further improvement in exhaust gas purification performance.
Further, as an exhaust gas purification device in which a DPF and a DeNox catalyst are combined, there is known one in which a contrivance is made in terms of exhaust gas flow, thereby achieving an overall reduction in size (Patent Document 1). In the exhaust gas purification device disclosed in Patent Document 1, a columnar DPF is arranged at the center, and an annular DeNox catalyst is arranged so as to surround the DPF. Exhaust gas is caused to flow in at one end of the DPF, and to flow out at the other end thereof. The flow of the exhaust gas caused to flow out is reversed, caused to flow in at the other end (on the same side as the other end of the DPF) of the DeNOx catalyst in the outer periphery, and caused to flow out at one end thereof (on the same side as one end of the DPF).
[Patent Document] U.S. Pat. No. 6,680,037